3 research outputs found
Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts
We report the reaction pathways and barriers for the oxygen reduction reaction (ORR) on platinum, both for gas phase and in solution, based on quantum mechanics calculations (PBE-DFT) on semi-infinite slabs. We find a new mechanism in solution: O<sub>2</sub> → 2O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.00 eV), O<sub>ad</sub> + H<sub>2</sub>O<sub>ad</sub> → 2OH<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.50 eV), OH<sub>ad</sub> + H<sub>ad</sub> → H<sub>2</sub>O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.24 eV), in which OH<sub>ad</sub> is formed by the hydration of surface O<sub>ad</sub>. For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O<sub>2</sub> is H<sub>ad</sub> + O<sub>2ad</sub> → HOO<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.30 eV) → HO<sub>ad</sub> + O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.12 eV) followed by O<sub>ad</sub> + H<sub>2</sub>O<sub>ad</sub> → 2OH<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.23 eV), OH<sub>ad</sub> + H<sub>ad</sub> → H<sub>2</sub>O<sub>ad</sub> (<i>E</i><sub>act</sub> = 0.14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for O<sub>ad</sub> hydration while providing hydrophobic conditions for the OH and H<sub>2</sub>O formation steps
Finding Correlations of the Oxygen Reduction Reaction Activity of Transition Metal Catalysts with Parameters Obtained from Quantum Mechanics
To facilitate a less empirical approach
to developing improved
catalysts, it is important to correlate catalytic performance to surrogate
properties that can be measured or predicted accurately and quickly,
allowing experimental synthesis and testing of catalysts to focus
on the most promising cases. Particularly hopeful is correlating catalysis
performance to the electronic density of states (DOS). Indeed, there
has been success in using just the center of the d-electron density,
which in some cases correlates linearly with oxygen atom chemisorption
energy, leading to a volcano plot for catalytic performance versus
“d-band center”. To test such concepts we calculated
the barriers and binding energies for the various reactions and intermediates
involved in the oxygen reduction reaction (ORR) for all 12 transition
metals in groups 8–11 (Fe–Cu columns). Our results show
that the oxygen binding energy can serve as a useful parameter in
describing the catalytic activity for pure metals, but it does not
necessarily correlate with the d-band center. In addition, we find
that the d-band center depends substantially on the calculation method
or the experimental setup, making it a much less reliable indicator
for ORR activity than the oxygen binding energy. We further examine
several surfaces of the same pure metals to evaluate how the d-band
center and oxygen binding energy depend on the surface
Density Functional Theory Study of Pt<sub>3</sub>M Alloy Surface Segregation with Adsorbed O/OH and Pt<sub>3</sub>Os as Catalysts for Oxygen Reduction Reaction
Using quantum mechanics calculations,
we have studied the segregation
energy with adsorbed O and OH for 28 Pt<sub>3</sub>M alloys, where
M is a transition metal. The calculations found surface segregation
to become energetically unfavorable for Pt<sub>3</sub>Co and Pt<sub>3</sub>Ni, as well as for the most other Pt binary alloys, in the
presence of adsorbed O and OH. However, Pt<sub>3</sub>Os and Pt<sub>3</sub>Ir remain surface segregated and show the best energy preference
among the alloys studied for both adsorbed species on the surface.
Binding energies of various oxygen reduction reaction (ORR) intermediates
on the Pt(111) and Pt<sub>3</sub>OsÂ(111) surfaces were calculated
and analyzed. Energy barriers for different ORR steps were computed
for Pt and Pt<sub>3</sub>Os catalysts, and the rate-determining steps
(RDS) were identified. It turns out that the RDS barrier for the Pt<sub>3</sub>Os alloy catalyst is lower than the corresponding barrier
for pure Pt. This result allows us to predict a better ORR performance
of Pt<sub>3</sub>Os compared to that of pure Pt